The present invention also provides a process for polymerizing ethylene or copolymerizing ethylene with C3–C8-α-olefins in such a reactor.
Gas-phase polymerization processes are today among the preferred processes for polymerizing ethylenically unsaturated monomers, in particular for the polymerization of ethylene, if desired in the presence of further unsaturated monomers. Polymerization processes in fluidized beds are moreover particularly economical.
Gas-phase fluidized-bed reactors for carrying out such processes have been known for a long time. The reactors which are customary today have many common structural features: they comprise, inter alia, a reactor space in the form of a vertical tube whose diameter increases in the upper part. In this calming zone, there is a lower gas flow as a result of the larger tube diameter and this limits the fluidized bed comprising small polymer particles. In addition, these reactors have a circulated gas line in which there are installed cooling units for removing the heat of polymerization, a compressor and, if desired, further elements such as a cyclone for removing fine polymer dust. Examples of such gas-phase fluidized-bed reactors are described, for example, in EP-A-0 202 076, EP-A-0 549 252 and EP-A-0 697 421.
These known gas-phase fluidized-bed reactors use gas distributor constructions configured as perforated plates, sometimes in combination with an upstream flow divider, to distribute the gas uniformly across the entire reactor cross section. All these constructions result, between the gas outlet orifices or drilled holes, in more or less extensive, depending on hole spacing, horizontal, planar surfaces on the upper side of the reactor bottom against which the gaseous reaction medium flows only to a limited extent. To avoid deposits of product on these surfaces, it is possible, as described in EP-A-0 173 261, to arrange roof-shaped deflector plates above the bottom plate in such a way that the reaction medium passes over the faces of the deflector plates and the surface of the bottom. An exclusively vertical flow of the reaction medium directly into the powder bed is thus not present with this arrangement.
However, the vertical blowing-out of the surface of the bottom appears to be important when restarting the fluidized bed, e.g. after a shutdown; in addition, roof-shaped deflector plates lead to a higher pressure drop than is necessary for producing a uniform fluidized bed and are therefore associated with unnecessary power input and energy consumption.
Likewise horizontal swirling of the reaction medium on the surface of the bottom is described in EP-A-0 512 147. This is achieved by gas flow orifices running at an angle to the surface of the gas distributor, but these are technically very complicated to manufacture and therefore have to be let into the bottom plate as individual pieces. The conspicuous length of the gas channels also makes it easier for the gas distributor plate to become blocked and makes it more difficult to clean.
In documents such as EP-A-549 252, EP-A-297 794 and EP-A-509 618, attempts are made to keep the polymer powder flowing at the surface of the bottom by means of gas distributor plates which are folded or angled inward. Blowing-off or -out of the surface of the bottom to avoid product deposits is restricted in these constructions. It is an object of the present invention to provide a gas-phase fluidized-bed reactor whose gas distributor plate is constructed such that the indicated disadvantages can be avoided and deposit formation at the upper side of the reactor bottom can be reduced in a simple way.